Under the mobile communication standards of the Third Generation Partnership Project (3GPP), Long Term Evolution (LTE) has been proposed for even faster communication. To robustly achieve increases in communication speed consequent to such improvements in communication standards, the amount of communication data handled by one terminal has increased.
To cope with such increases in communication volumes, a small-scale base station (HeNB: Home eNodeB) having a cell (femtocell) of a small radius is provided in the given radius of the macrocell of a base station (eNodeB (eNB)). This small-scale base station is provided at hotspots where many base stations performing communication are present and in dead zones where signals do not reach to improve the communication state in these areas. This small-scale base station has a high degree of freedom in terms of installation location and can be easily installed where necessary.
Among femtocells is a cell that only terminals belonging to a closed subscriber group (CSG), which is a group of pre-registered terminals, can access called a CSG cell. CSG cells use system information block type 1 (SIB1) included in broadcast information and notify nearby terminals of identification information (csg-Identity) of the CSG cell, whereby the terminals can attempt to connect to a connectable CSG cell based on the identification information.
Further, since femtocells assume the use of, as a network-side line (backhaul), a broadband line under individual or corporate contract, a majority of femtocells are assumed to be operated as CSG cells.
Communication with counterpart terminals is subject to interference by signals of adjacent cells. Interference between adjacent cells causes drops in communication quality and therefore, various types of techniques have been proposed to prevent interference. As one technique to reduce interference under the 3GPP (Release10) communication standard, enhanced intercell interference coordination (eICIC) has been specified. Based on 1-ms subframes as a unit of time, an eNB manages radio resources used in cellular communication. A cell performing normal communication transmits and receives data at each subframe. Under eICIC, if a terminal connected to cell A receives interference from cell B, which is adjacent to cell A, the base station (eNB2) of cell B implements an almost blank subframe (ABS) where for a portion of the subframes of cell B, substantially no data is transmitted or received. As a result, cell A and the terminal communicate during the implementation of ABS by cell B, whereby interference can be reduced. However, for cell A and the terminal to communication at the timing of ABS implementation by cell B, the base station (eNB1) of cell A has to know the timing of ABS by cell B.
Usually, adjacent base stations are connected by an X2 interface. The base station (eNB1) specifies ABS information in a message (parameter Invoke Indication of LOAD INFORMATION) on the X2 interface whereby, the base station (eNB1) requests the base station (eNB2) for ABS parameters of cell B. Having received the request, the base station (eNB2) sets given ABS parameters for cell B in a message (parameter ABS Information of LOAD INFORMATION) and sends the message to the base station (eNB1). The base station (eNB1) receives the message and communicates with the terminal in cell A, based on the received ABS parameters.
The base station (eNB2) can use a message (RESOURCE STATUS REQUEST) on the X2 interface to inquire about the utilization state of the ABS reported to the base station (eNB1). The base station (eNB1) can use a message (RESOURCE STATUS RESPONSE, RESOURCE STATUS FAILURE and RESOURCE STATUS UPDATE) on the X2 interface to reply to the base station (eNB2) concerning the inquiry. Under the communication standard 3GPP, an ABS pattern to be used by a cell is notified from a management apparatus, such as an eNodeB Management System (eMS), to a base station (eNB) and can also be specified.
Although an ABS is a subframe carrying substantially no information, an ABS includes a cell reference signal (CRS), which is information for measuring cell reception quality. Further, an ABS includes a paging, positioning reference signal (PRS) for synchronization as well as reporting and managing packet reception, a primary synchronization signal (PSS), a secondary synchronization signal (SSS), and a physical broadcast channel (PBCH), enabling transmission of related information. The timing of a subframe to be an ABS is presently specified as a recursive pattern of 40 ms cycles, and thus, given ABS patterns using 40 subframes are created. In the description hereinafter, an ABS, in addition to the ABS above, is a multimedia broadcast-multicast service single frequency network (MBSFN) subframe that can be used as an ABS, and includes subframes that compared to normal subframes are extremely poor at suppressing transmission output.
For cases where a microcell such as a femtocell is arranged in a macrocell, techniques have been proposed that prevent interference between the respective base stations of the macrocell and femtocell. According to one such technique, a terminal located in a microcell of a macrocell transmits interference power information to the macrocell base station and the macrocell base station notifies the concerned microcell base station of the information, whereby the microcell base station uses a transmission frame capable of reducing the interference (see, for example, Japanese Laid-Open Patent Publication No. 2012-5079).
According to another technique, a microcell base station detects uplink interference of a terminal connected to a macrocell base station and notifies the macrocell base station of the interference pattern. The macrocell base station notifies the microcell base station of scheduling information for a user terminal, based on the interference pattern; and based on the scheduling information, the microcell base station, performs scheduling of the terminal (see, for example, Japanese Laid-Open Patent Publication No. 2012-5086).
A further technique uses ICIC information over an X2 interface or an S1 interface between a macrocell base station and a microcell base station; by a high interference indicator (HII), identifies a frequency resource subject to interference; and performs resource reservation (see, for example, Published Japanese-Translation of PCT Application, Publication No. 2011-518519).
Another technique searches for a CSG cell adjacent to a terminal WTRU, reads a master information block (MIB) and a system information block (SIB), and determines whether intermittent reception (DRX gap) is sufficient for this reading. If the DRX gap is insufficient, detuning from the cell is performed and the MIB and the SIB are read (see, for example, Published Japanese-Translation of PCT Application, Publication No. 2011-518471).
With respect to the base station of a CSG cell and of a non-CSG cell, a further technique issues a CSG-ID to a terminal permitted to use the CSG cell, thereby enabling the terminal to access the base station of the CSG cell (see, for example, Japanese Laid-Open Patent Publication No. 2010-136337).
Nonetheless, compared to the macrocell base station of a macrocell in which a femtocell is located, a small-scale base station (HeNB) having a cell of a small diameter can be installed dynamically without fixing its location and on this point, differs from base stations (eNB) having macrocells that are fixed in urban areas. Thus, proper management of the installation location of small-scale base stations is difficult and at an eNB and a HeNB, the maintenance of accurately set adjacent cell information related to a femtocell is difficult. In addition, configuration may be such that a small-scale base station does not have an X2 interface for exchanging information with an adjacent cell.
FIGS. 16A and 16B are diagrams for describing a problem accompanying movement of a femtocell. As depicted in FIGS. 16A and 16B, under the 3GPP communication standards, an X2 interface is limited to only opposing HeNBs. However, a HeNB typically provided in a household or place of business has a cell range of several dozen meters. Therefore, the HeNB of a femtocell 1601 depicted in FIG. 16A may be frequently moved to an installation position beyond the range of the cell as depicted in FIG. 16B. In such a case, even if the X2 interface is temporarily provided in the femtocell, it is difficult to accurately discern the adjacent cells.
In residential apartment complexes and business complexes, when an HeNB (cell B) is installed near another HeNB (cell A), communication with terminal A in cell A interferes with the communication with terminal B in cell B. Conversely, the communication with terminal B in cell B interferes with the communication with terminal A in cell A. As described, a femtocell is assumed to be operated as a CSG cell in most cases and therefore, when an adjacent cell is a femtocell, even if the interference is great, a switching, by handover, of connection to the cell causing the interference may not occur.
Concerning such interference, a preventative measure applying ABS is assumed. However, as described, when HeNBs have no X2 interface, the HeNBs cannot notify one another of each other's respective ABS pattern via the X2 interface. As a result, cell A and cell B can conceivably apply ABS concurrently, hindering effective prevention of interference. When there are 3 or more adjacent CSG cells, the timing of the ABS between the CSG cells becomes further overlapped and even if a management apparatus such as a Home eNodeB Management System (HeMS) is configured to notify each CSG cell of the ABS patterns, as described, a cell recognized by the HeMS to be adjacent may not coincide with an actual HeNB that is physically adjacent, arising in a problem that interference cannot be prevented.